This invention relates generally to a method of producing a semiconductor device and, more particularly, to a method for producing a semiconductor device equipped with electrodes and interconnections consisting of tungsten, molybdenum or their silicides.
As is well known in the art, polycrystalline silicon has been used widely as a material for electrodes and interconnections of a conventional semiconductor device.
Polycrystalline silicon has been used for the following reasons: In order to miniaturize an MIS (e.g., MOS) transistor, it is inevitable to employ so-called "self-alignment" techniques which form source and drain by ion implantation using the gate electrode as the mask. After ion-implantation is completed, however, annealing at high temperatures must be made to remove distortion of the source and drain region that has developed due to the ion-implantation.
Accordingly, to produce a miniature MOS transistor by self-alignment, the gate electrode must be made of a material which can withstand the heat-treatment at high temperatures, and polycrystalline silicon having a high melting point has replaced aluminum that was used widely and previously.
The disadvantage of this polycrystalline silicon is, however, that its electric resistance is greater than metals. Since higher integration and miniaturization of semiconductor devices has made a rapid process in recent years, the width of the electrode or interconnection becomes extremely small. Accordingly, if polycrystalline silicon is used, the resistance of the electrode or interconnection does not become sufficiently low and it is difficult to produce a miniature semiconductor device having high characteristics.
To solve this problem, the use of tungsten, molybdenum or their silicides has been proposed in place of polycrystalline silicon so as to form the electrode and the interconnection. Since tungsten, molybdenum or their silicides have a high melting point, they can withstand annealing at high temperatures and, moreover, since their electric resistance is by far lower than that of polycrystalline silicon, the problem described above that occurs when polycrystalline silicon is used does not develop even when the width of the electrode or interconnection is extremely small.
However, tungsten and molybdenum have the problem than they are more easily oxidized than silicon. When heat-treatment is carried out at about 300.degree. C. or above in an oxidizing atmosphere, therefore, they are rapidly oxidized, disloged or peel off from the substrate.
An insulating film (e.g., SiO.sub.2 film) deposited on a semiconductor substrate is demaged or contaminated if an insulating film becomes thin or ion-implantation is effected using the gate as the mask to form the source and drain during the fabrication of an MOS transistor. Accordingly, the damaged or contaminated insulating film must be removed by etching after completion of gate formation and ion-implantation, and heat-treatment is carried out in an oxidizing atmosphere to regrow an SiO.sub.2 film on the semiconductor substrate. This process is carried out generally and widely and is an indispensable step to form a high reliability MOS transistor. (This process or treatment will hereinafter be referred to as "light oxidation".)
When polycrystalline silicon is used as the gate electrode and the interconnection, light oxidation described above can be carried out smoothly without any problem, but since tungsten and molybdenum are extremely oxidizable as described earlier, the gate electrode and the interconnection are extremely easily oxidized when tungsten or molybdenum is used as the material, so that the semiconductor devices having high reliability and high integration density cannot be produced.